High-voltage flywheel energy storage system

ABSTRACT

A high-voltage flywheel energy storage system to prevent ionization, plasma formation, and electrical are discharge and corresponding method are provided. The high-voltage flywheel energy storage system prevents ionization, plasma formation, and electrical are discharge by isolating the motor windings and motor end windings from the partial vacuum environment existing in the flywheel housing.

BACKGROUND

1. Field

Embodiments of the present invention relate to energy storage systems.In particular, high-voltage flywheel energy storage systems storeelectrical energy as kinetic energy in a rotating flywheel. The storedenergy can also be released from the high-voltage flywheel energystorage system.

2. Background

Large-scale energy storage has the potential to help modernizeelectrical power distribution. Energy storage can help manageintermittent renewable energy generation, electricity load shifting,black-start capabilities, electricity price fluctuations, and back-uppower supply.

There are currently several large-scale energy storage technologies thatattempt to address these modernization challenges facing the energystorage industry, including: advanced batteries; electrochemicalcapacitors (EC); pumped hydro; compressed air; and flywheel energystorage systems.

Due to low costs associated with lead acid batteries, they have been apopular choice for power quality and uninterruptable power supply (UPS)applications. However, the effectiveness of lead acid batteries forlarge-scale applications is limited by a short battery life cycle and avariable discharge rate. Li-ion batteries are often seen as analternative or replacement for lead acid due to a longer life cycle. Theeffectiveness of Li-ion batteries for large scale energy storage islimited, however, by a high manufacturing cost and by security concernsassociated with large-scale implementation. Metal-Air batteries are themost compact and potentially the least expensive battery to manufacture.However, the effectiveness of Metal-Air batteries is limited by a veryshort life cycle and low efficiency (e.g., approximately 50%).Sodium-sulphur (NaS) battery technology has shown promise as a solutionfor large-scale implementation. NaS batteries have high energy densitybut require high operating temperatures and have a relatively short lifespan. Battery technologies typically have an average AC to AC round-tripefficiency of approximately 64%. And, electrochemical batterytechnologies generally have a usable life that is degraded by the numberof charge/discharge cycles.

Electrochemical capacitors (EC) are energy storage devices that havelonger life cycles and are more powerful than lead-acid batteries.However, it is not feasible to implement ECs on large-scale projects dueto their high cost and low energy density.

Conventional pumped hydro as an energy storage technology uses two waterreservoirs that are separated vertically. An energy potential due togravity is associated with the energy of the water travelling from theelevation of higher potential energy to the elevation of lower potentialenergy. During off-peak hours, electrical power is used to pump waterfrom the lower reservoir to the upper reservoir. As demand forelectrical energy increases, the water flow is reversed to generateelectricity. Pumped hydro offers beneficial energy management andfrequency regulation, but requires unique site requirements and largeupfront capital costs.

Compressed air energy storage (CAES) uses a combination of compressedair and natural gas. A motor pushes compressed air into an undergroundcavern at off-peak times. During on-peak times, compressed air is usedin combination with gas to power a turbine power plant. A CAES usesroughly 40% as much gas as a natural gas power plant and similarly topumped hydro, requires unique site requirements and large upfrontcapital costs.

Flywheel energy storage systems have emerged as an alternative to theabove-identified energy storage technologies. Flywheel energy storagesystems are currently used in two primary commercial applications: UPSand power frequency regulation (FR). Both UPS and FR require extremelyquick charge and discharge times that are measured in seconds andfractions of seconds. Flywheel technologies have high reliability, longservice life, extremely low maintenance costs, high power capability,and environmental friendliness. Flywheel energy storage systems storeenergy in a rotating flywheel that is supported by a low frictionbearing system inside a housing. A connected motor/generator acceleratesthe flywheel for storing inputted electrical energy, and decelerates theflywheel for retrieving this energy. Power electronics maintain the flowof energy into and out of the system to mitigate power interruptions, oralternatively, manage peak loads.

Often, the rotating flywheel and motor/generator rotor operate in atleast a partial vacuum to reduce windage losses due to drag forcesacting on the flywheel. In the case of high-voltage flywheel energystorage systems, power supplies present a raised risk of ionization andplasma formation on the windings in the motor/generator. Such plasmaformation can lead to electric are discharge. This is especially truewhen the motor/generator operates in a partial vacuum environment.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention obviate or mitigate plasma formation inhigh-voltage flywheel energy storage systems and the associated risk ofelectrical are discharge.

In one aspect of the invention, a high-voltage flywheel energy storagesystem is provided including a motor/generator and a flywheel located ina flywheel housing. The motor/generator can include a rotor connected tothe flywheel, a stator including a motor winding, and an ionizationavoidance barrier that prevents plasma formation on the motor winding byisolating the motor winding from a reduced interior pressure of theflywheel housing.

In another aspect of the invention, a method for preventing plasmaformation in a high-voltage flywheel energy storage system includesproviding a flywheel in a flywheel housing, drawing a partial vacuum inthe flywheel housing, providing a motor/generator, the motor/generatorhaving a stator including a motor winding and a rotor connected to theflywheel, and preventing plasma formation on the motor winding byisolating the motor winding from the partial vacuum in the flywheelhousing.

In another aspect of the invention, a method for reducing electric aredischarge in a high-voltage flywheel energy storage system includesproviding a flywheel in a flywheel housing, providing a motor/generatorhaving a rotor connected to the flywheel and a stator including a motorwinding, and preventing electric are discharge from the motor winding byreducing the interior pressure of the motor/generator housing belowapproximately 1×10⁻³ Torr.

In another aspect of the invention, a method for reducing plasmaformation in a high-voltage flywheel energy storage system includesproviding a flywheel in a flywheel housing, drawing a partial vacuum inthe flywheel housing, providing a motor/generator having a rotorconnected to the flywheel and a stator including a motor winding in amotor/generator housing, isolating the motor winding from the partialvacuum in the flywheel housing, and drawing a deep vacuum in themotor/generator housing, where isolating the motor winding and drawing adeep vacuum prevents ionization and plasma formation in the motorwinding.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate embodiments of the present inventionand, together with the description, further serve to explain theprinciples of the invention and to enable a person skilled in therelevant art(s) to make and use the invention.

FIG. 1 is a front perspective view of a high-voltage flywheel energystorage system according to an aspect of the invention.

FIG. 2 is a cross-sectional view along the line A-A of FIG. 1 accordingto an aspect of the invention.

FIG. 3 is partial cross-sectional view along the line A-A of FIG. 1according to an aspect of the invention.

FIG. 4 is partial cross-sectional view along the line A-A of FIG. 1according to an aspect of the invention.

FIG. 5 is a partial cross-sectional view along the line B-B of FIG. 1according to an aspect of the invention.

FIG. 6 is a diagram showing Paschen's Law in air with breakdown voltageon the y-axis and gap distance on the x-axis.

Features and advantages of the embodiments will become more apparentfrom the detailed description set forth below when taken in conjunctionwith the drawings, in which like reference characters identifycorresponding elements throughout.

DETAILED DESCRIPTION

It is noted that references in the specification to “one embodiment,”“an embodiment,” “an example embodiment,” etc., indicate that theembodiment described may include a particular feature, structure, orcharacteristic, but every embodiment may not necessarily include theparticular feature, structure, or characteristic. Moreover, such phrasesare not necessarily referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with an embodiment, it is submitted that it is within theknowledge of one skilled in the art to effect such feature, structure,or characteristic in connection with other embodiments, whether or notexplicitly described.

FIG. 1 is a perspective view of a high-voltage flywheel energy storagesystem 10, according to one aspect of the invention. As better shown inFIG. 2, high-voltage flywheel energy storage system 10 can include aflywheel housing 100 and a motor/generator 200. For the purposes of thisdescription, high-voltage refers to exposing portions of themotor/generator 200 to voltages above approximately 300 volts. Flywheelhousing 100 can include vacuum envelope 110 and vacuum enclosure 101which can be supported at a first end by bottom plate 310 and at asecond end by top plate 300. Flywheel housing 100 can contain arotatably mounted flywheel 130. To reduce windage losses due to dragforces acting on flywheel 130, the interior pressure within flywheelhousing 100 can be reduced. For example, the interior pressure offlywheel housing 100 can be at least a partial vacuum. So, flywheelhousing 100 includes a vacuum port 120 that can extend through vacuumenclosure 101. A vacuum pump (not shown) can be connected to vacuum port120 to remove air, gas, or vapor from vacuum enclosure 101 and vacuumenvelope 110 to reduce the interior pressure within flywheel housing100. In one aspect of the invention, the interior pressure withinflywheel housing 100 can be approximately 0.01 Torr. In another aspectof the invention, the interior pressure within flywheel housing 100 canbe below approximately 0.01 Torr. In another aspect of the invention,the interior pressure within flywheel housing 100 can be a deep vacuumat approximately 1×10⁻³ Torr. In another aspect of the invention, theinterior pressure within flywheel housing 100 can be a deep vacuum belowapproximately 1×10⁻³ Torr. In another aspect of the invention, theinterior pressure within flywheel housing 100 can be a deep vacuum atapproximately 1×10⁻⁴ Torr. In another aspect of the invention, theinterior pressure within flywheel housing 100 can be a deep vacuum belowapproximately 1×10⁻⁴ Torr. In another aspect of the invention, theinterior pressure within flywheel housing 100 can be a deep vacuum belowapproximately 1×10⁻⁶ Torr. In another aspect of the invention, theinterior pressure within flywheel housing 100 can be a deep vacuumbetween approximately 1×10⁻⁴ Torr and approximately 1×10⁻⁶ Torr. Inanother aspect of the invention, the interior pressure within flywheelhousing 100 can be a deep vacuum between approximately 1×10⁻³ Torr andapproximately 1×10⁻⁶ Torr.

Flywheel 130 can also include a flywheel rotor 132 that extends intomotor/generator 200. In one aspect of the invention, flywheel 130 andflywheel rotor 132 are made from a ferromagnetic material. Flywheel 130and flywheel rotor 132 can be integrally formed. In an alternate aspect,flywheel 130 and flywheel rotor 132 can be fabricated separately andcoupled during assembly of high-voltage flywheel energy storage system10. Energy can be stored in flywheel 130 and flywheel rotor 132 in theform of kinetic energy.

Flywheel 130 and flywheel rotor 132 can be supported at a first end bylower mechanical bearing 410 and at a second end by upper mechanicalbearing 400. Magnetic bearing 500 can act between housing top plate 300and flywheel 130 to support a significant portion of the weight offlywheel 130. Thus, magnetic bearing 500 can relieve lower mechanicalbearing 410 and upper mechanical bearing 400 from axial loading. In oneaspect of the invention, magnetic bearing 500 includes a permanentmagnet.

Motor/generator 200 can include motor frame 201 positioned betweenbottom plate 310 and motor end 202. Motor frame 201 and motor end cap202 form a motor/generator housing which can be detachably attached toflywheel housing 100 through screws, bolts, or other suitable attachmentmeans. In one aspect, motor/generator 200 can be an induction typemotor/generator. More particularly, motor/generator 200 can be athree-phase induction type unit. Motor/generator 200 can also be afour-pole motor and can operate at approximately 400 Hz. Motor/generator200 can include motor stator 210 and motor rotor 220. Motor stator 210can include motor windings 230 and motor end windings 232. As shown inFIG. 5, stator 210 can include stator openings 212. Motor windings 230extend through stator openings 212. Electrical cable connections tomotor/generator 200 (not shown) can be made through motor end cap 202.The electrical cable connections provide power to motor windings 230including motor end windings 232. Gap 260 can also be provided betweenmotor stator 210 and motor rotor 220 to permit rotation of motor rotor220.

In one aspect, motor frame 201 can include motor cooling channels 240 toprovide liquid cooling of motor/generator 200. Motor cooling channels240 can encircle the outer surface of motor frame 201. The coolant flowcan be provided via an external pump or natural convection to removewaste heat from motor/generator 200.

Flywheel rotor 132 can be coupled to motor rotor 220. Motor/generator200 can be capable of rotating flywheel 130 at high speed, betweenapproximately 10,000 RPM and approximately 20,000 RPM. As flywheel 130accelerates, the energy supplied by motor/generator 200 can be stored askinetic energy. Upon reaching maximum speed, the electrical power can bedisconnected. If the power is disconnected, or if additional electricalenergy is required by the power grid, motor/generator 200 can beswitched to a generating mode and the energy stored in flywheel 130 candrive the generator to supply electrical power. In some embodiments, thestorage capacity of the high-voltage flywheel energy storage system 10can be approximately 20 kWh. In an alternate embodiment, the storagecapacity of the high-voltage flywheel energy storage system 10 can beapproximately 50 kWh. In a further embodiment, the storage capacity ofthe high-voltage flywheel energy storage system 10 can be fromapproximately 1 kWh to approximately 50 kWh. The storage capacity of thehigh-voltage flywheel energy storage system 10 can also be greater thanapproximately 50 kWh. The energy storage is a function of the diameterand material density of flywheel 130 and the speed at which flywheel 130is rotated.

In operation, power can be supplied to and from motor/generator 200 andmotor rotor 220 through power conversion electronics (not shown) whichcan include a variable frequency drive (VFD). The VFD can be connectedto the power grid, e.g. at 480 volts AC, 3 phase, and 60 Hz. In themotoring process of flywheel energy storage system 10, the VFD can drawpower from the grid, rectify the AC power to DC, e.g. at 750 volts DC,and then reroute the DC power to a DC bus (not shown). The DC bus cansupply power to a second VFD. The second VFD can create an AC voltagefrom approximately 0 volts AC to approximately 480 volts AC at avariable frequency of approximately 0 Hz to approximately 400 Hz. Therotational speed of motor rotor 220 in motor/generator 200 can becontrolled by varying the frequency and voltage. For example, the secondVFD can increase the speed of motor rotor 220 by increasing thefrequency and voltage supplied to motor/generator 200. The second VFDreassembles the AC sine wave by switching the DC voltage on and off veryquickly. As a result, the VFD produces a sine wave approximation thatcan have a peak to peak spike voltage of approximately two times the DCbus voltage. Thus, the motor windings 230 can experience peak voltagesin of up to approximately 2000 volts. These high voltages increase therisk of ionization and plasma formation in motor/generator 200.

Alternatively, motor/generator 200 can behave as a generator byutilizing inertial energy of motor rotor 220 and flywheel rotor 132 tocreate energy that can be supplied back to the grid through the powerconversion electronics. In the generating process of flywheel energystorage system 10, motor/generator 200 can supply AC power to the secondVFD which can rectify the AC power to DC for routing along the DC bus.The DC bus can then supply the power to the first VFD which can convertthe power back into AC for transfer to the grid.

Motor/generator 200 can operate at high generator voltages, for exampleabove approximately 300 volts. In one aspect of the invention,motor/generator 200, including motor windings 230 and motor end windings232, can be exposed to voltages from approximately 300 volts toapproximately 2,000 volts. In particular, motor/generator 200, includingmotor windings 230 and motor end windings 232, can be exposed tovoltages between approximately 300 volts and approximately 690 volts.And, motor/generator 200, including motor windings 230 and motor endwindings 232, can be exposed to a number of voltages, includingapproximately 300 volts; approximately 400 volts; approximately 440volts; approximately 460 volts; approximately 480 volts; approximately600 volts; and approximately 690 volts.

The exposure of motor/generator 200, including motor windings 230 andmotor end windings 232 to high voltage presents a risk of ionization andplasma formation. Plasma formation can be caused by the presence of ahigh-voltage electric field around energized motor winding conductors.If the electric field extends into a region where air, gas, or vapormolecules are present in a partial vacuum environment, ionization of themolecules can occur resulting in plasma formation. The hot plasma canburn through the motor winding conductor insulation and can cause aground fault and/or permanent damage to motor windings 230 and flywheelenergy storage system 10 leading to premature equipment failure. Plasmaformation can lead to electric are discharge. Electric are discharge canbe harmful to motor windings 230 and flywheel energy storage system 10and can cause power loss, audible noise, electromagnetic interference,and insulation damage

As discussed above, flywheel housing can operate in a partial vacuumenvironment. The coupling of flywheel 130 and flywheel rotor 132 tomotor rotor 220 results in a fluid channel forming between flywheelhousing 100 and motor/generator 200. Thus, motor windings 230 and motorend windings 232 can be exposed to the partial vacuum in flywheelhousing 100. FIG. 6 depicts the breakdown voltage required to form anelectric are on the y-axis and the product of pressure and distance onthe x-axis. The curve illustrates the variation of breakdown voltage inair versus pressure and gap distance. The relationship between breakdownvoltage and air pressure for a particular gap distance is governed byPaschen's Law:

${V = \frac{a({pd})}{\left( {{{ln}\;({pd})} + b} \right)}},$where V is breakdown voltage; p is pressure; d is the gap distance; anda and b are constants that depend on the gas composition. As shown inFIG. 6, Paschen's curve passes through a minimum breakdown voltage inair of approximately 327 volts at 1 atmosphere of pressure. So, at thisvoltage and pressure, electrical arcing could occur in a flywheel energystorage system over a gap distance of approximately 7.5 μm betweenelectrical components, e.g. from a motor winding to ground or to anadjacent motor winding. As shown by the small 7.5 μm gap, high-voltageflywheel energy storage system 10 is not at risk for electrical arcingat ambient air pressure, 1 atm.

At pressures below atmosphere, high-voltage flywheel energy storagesystems, such as high-voltage flywheel energy storage system 10, have asignificant risk of ionization, plasma formation, and electrical arcingfrom the motor windings 230 and motor end windings 232 to othercomponents within motor/generator 200. The present invention isolatesmotor windings 230 and motor end windings 232 from the partial vacuum inflywheel housing 100 to prevent ionization, plasma formation, andelectrical are discharge.

In one aspect of the invention, ionization, plasma formation, andelectrical are discharge can be prevented in motor/generator 200 byutilizing motor winding encapsulant 250 to isolate motor winding 230 andmotor end winding 232 from the partial vacuum in flywheel housing 100.FIG. 3 depicts motor winding 230 and motor end winding 232 prior to theaddition of motor winding encapsulant 250. As shown in FIGS. 4-5, motorwinding encapsulant 250 completely surrounds, impregnates, and sealsmotor windings 230 and motor end windings 232. Motor winding encapsulant250 eliminates all air, gas, or vapor voids surrounding motor windings230 in stator openings 212 and all air, gas, or vapor voids surroundingmotor end windings 232. Because motor windings 230 and motor endwindings 232 are completely isolated from all gases including air orvapor, ionization cannot occur, and plasma and electric arcs cannotform. Motor/generator 200 can thus prevent ionization, plasma formation,and electrical are discharge at any voltage, regardless of the interiorpressure of flywheel housing 100.

Motor winding encapsulant 250 can be added to motor/generator 200according to the Ultra-Sealed Winding™ process developed by DreisilkerElectric Motors. Motor winding encapsulant 250 can also be added tomotor/generator 200 according to the field winding encapsulation processdescribed in U.S. Pat. Nos. 5,759,589 and 5,939,102 to Georges, Jr., thedisclosures of which are incorporated herein by reference in theirentirety.

In another aspect of the invention, a bottom portion of motor/generator200 between motor winding encapsulant 250 and motor end cap 202 can beat ambient air pressure. ionization, plasma formation, and electricalare discharge can be prevented in motor/generator 200 by isolating theelectrical cable connections to motor/generator 200 (not shown) from thepartial vacuum in flywheel housing 100 through positioning theconnections in this portion of motor/generator 200 at ambient airpressure. In one aspect of the invention, o-ring seal 204 can beprovided to limit the partial vacuum to the interior of motor/generator200 allowing the interior pressure of the bottom portion ofmotor/generator 200 to be at ambient air pressure.

In another aspect of the invention, ionization, plasma formation, andelectrical are discharge can be prevented in motor/generator 200 byisolating motor winding 230 and motor end winding 232 from the partialvacuum in flywheel housing 100. For example, motor/generator 200 orflywheel housing 100 can include a seal to prevent the partial vacuum inflywheel housing 100 from extending into motor/generator 200. In oneaspect, the seal could be a non-contact seal. In additional aspects, amagnetic liquid rotary seal, a labyrinth seal, or any other mechanicalor fluidic seal. Through such a seal, flywheel housing 100 can operateat a partial vacuum interior pressure while motor/generator 200 canremain at ambient air pressure thus removing the risk ionization, plasmaformation, and electrical are discharge in motor/generator 200.

In another aspect of the invention, electric are discharge can beprevented in motor/generator 200 by operating high-voltage flywheelenergy storage system 10 at a deep vacuum, for example at or belowapproximately 1×10⁻³ Torr, in both flywheel housing 100 andmotor/generator 200. Operating high-voltage flywheel energy storagesystem 10 at a deep vacuum also mitigates the adverse effects ofionization and plasma formation. In another aspect of the invention, theinterior pressure within flywheel housing 100 and motor/generator 200can be a deep vacuum at or below approximately 1×10⁻⁴ Torr. In anotheraspect of the invention, the interior pressure within flywheel housing100 and motor/generator 200 can be a deep vacuum at or belowapproximately 1×10⁻⁶ Torr. In another aspect of the invention, theinterior pressure within flywheel housing 100 can be a deep vacuumbetween approximately 1×10⁻³ Torr and approximately 1×10⁻⁶ Torr. Inanother aspect of the invention, the interior pressure within flywheelhousing 100 can be a deep vacuum between approximately 1×10⁻⁴ Torr andapproximately 1×10⁻⁶ Torr. Creating a deep vacuum within thehigh-voltage flywheel energy storage system can prevent stable electricarcs from forming. For example, at 1×10⁻⁷ Torr and 440 volts, anelectric are would occur at a gap distance between approximately 30 mand 152 m. This distance is too long for an electric are to stably form.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A high-voltage flywheel energy storage systemcomprising: a flywheel located in a flywheel housing, wherein aninterior pressure of the flywheel housing is reduced below atmosphericpressure; a motor/generator retained in a motor/generator housingadjacent and exterior to the flywheel housing, the motor/generatorcomprising: a rotor connected to the flywheel, and a stator including amotor winding; and an ionization avoidance barrier that prevents plasmaformation in the motor winding by isolating the motor winding from thereduced interior pressure of the flywheel housing.
 2. The system ofclaim 1, wherein an interior pressure of a portion of themotor/generator housing is equal to the interior pressure of theflywheel housing.
 3. The system of claim 2, wherein the rotor resides inthe portion of the motor/generator housing.
 4. The system of claim 2,wherein a fluid passageway extends between the flywheel housing and theportion of the motor/generator housing.
 5. A high-voltage flywheelenergy storage system comprising: a flywheel located in a flywheelhousing, wherein an interior pressure of the flywheel housing is reducedbelow atmospheric pressure; a motor/generator retained in amotor/generator housing adjacent the flywheel housing, themotor/generator comprising: a rotor connected to the flywheel, and astator including a motor winding; and an ionization avoidance barrierthat prevents plasma formation in the motor winding by isolating themotor winding from the reduced interior pressure of the flywheelhousing, wherein an interior pressure of a portion of themotor/generator housing is ambient air pressure.
 6. The system of claim5, wherein an interior pressure of a second portion of themotor/generator housing is equal to the interior pressure of theflywheel housing.
 7. The system of claim 6, wherein the rotor resides inthe second portion of the motor/generator housing.
 8. The system ofclaim 6, wherein a fluid passageway extends between the flywheel housingand the second portion of the motor/generator housing.